The present invention relates to a support for a lithographic printing plate and a presensitized plate. More particularly, the invention relates to a positive working presensitized plate having a photosensitive layer that can become alkali-soluble by photothermal conversion by a laser beam, and a support for a lithographic printing plate used for the same.
With development of image formation technology in recent years, it has come to be possible to perform direct plate making by scanning narrow focused laser beams on the printing plate to form a manuscript of letters, images and the like directly on the plate without using a film manuscript.
In a presensitized plate of a so-called thermal positive working type for causing photothermal conversion in a photosensitive layer by laser beam irradiation to make the photosensitive layer alkali soluble, and thus forming a positive image, a subtle change in interaction of binder molecules contained in the photosensitive layer by laser beam exposure is utilized as an image forming principle. Accordingly, a difference in ON/OFF levels of alkaline solubility between exposed and unexposed portions is reduced. Therefore, for the purpose of obtaining clear discrimination to be put to practical use, use has been made of means for forming a photosensitive layer structure by providing a surface slightly soluble layer in developer as an uppermost layer of the photosensitive layer, and suppressing developer solubility of the unexposed portion.
However, when the surface slightly soluble layer is damaged for some reason, even a portion intended as an image area is made easily soluble in the developer. In other words, a printing plate produced is damaged very easily from a practical standpoint. Thus, scratch-shaped non-image portion is brought about by subtle contacts such as clashing in handling of the printing plate, subtle rubbing of an interleaving sheet, contact of fingers with the plate surface or the like.
For example, in the presensitized plate, paper called an interleaving sheet for protecting the plate surface is normally provided on the surface of the photosensitive layer. This interleaving sheet is electrostatically adsorbed on the plate surface, thereby becoming difficult to be peeled off. At present, automatic feeding of the presensitized plate by a machine is generally carried out, and the interleaving sheet adsorbed on the plate surface is also removed mechanically. In this case, however, friction between the interleaving sheet and the photosensitive layer may cause scratching.
Therefore, the above-described presensitized plate of the thermal positive working type is still difficult to be handled in printing plate work. For the purpose of improving the tendency to be damaged, a layer of fluorine-containing surfactant or wax agent has been provided on the surface of the photosensitive layer to reduce a friction coefficient. However, no satisfactory measures have been taken.
Furthermore, the above-described scratch-like non-image portion caused by contact or the like has also been a problem even in the case of a presensitized plate provided with a photosensitive layer which doesn""t have a surface slightly soluble layer.
Objects of the present invention are to provide a positive working presensitized plate of a thermal type, which has damage resistance, and which is handled easily in conventional operation, high in sensitivity and excellent in press life when used as a lithographic printing plate, and to provide a support for a lithographic printing plate, which is suitably used for the same.
The inventors conducted serious studies in order to achieve the foregoing objects, and accordingly completed a support for a lithographic printing plate of a first aspect of the present invention.
That is, the first aspect of the present invention provides a support for a lithographic printing plate obtained by performing graining treatment, alkali etching treatment and anodizing treatment on an aluminum plate, wherein a ratio of a real area of a surface thereof to an apparent area of the surface set larger by 1.3 to 1.8 times, comprising a pit having an average diameter of 0.3 to 1.0 xcexcm and a micro grained structure inside (also referred to as xe2x80x9cgrained structure with small undulationxe2x80x9d hereinafter) on the surface, wherein a ratio of an apparent area of the pits to the apparent area of the surface is 90% or more.
In this case, xe2x80x9cratio of a real area of a surface thereof to an apparent area of the surfacexe2x80x9d means a value obtained by dividing a real area of a surface of the support for the lithographic printing plate by an apparent area of the surface, wherein the real area of the surface includes a pit surface area but not a surface area of the pit micro grained structure, while the apparent area represented by an area of a drawing of projecting the surface of the support for the lithographic printing plate on a surface parallel to the support. Specifically, when a surface shape of the support for the lithographic printing plate is measured by using an atomic force microscope (AFM) under conditions of horizontal (X, Y) resolution 0.1 xcexcm, and a measuring area of 100 xcexcm-square, a surface area obtained by an approximate three-point method is set as a real area, an upper projected area is set as an apparent area and, then, it can be obtained by dividing the real area with the apparent area.
Moreover, xe2x80x9cratio of an apparent area of the pits to the apparent area of the surfacexe2x80x9d means a value obtained by dividing a pit apparent area represented by an area of a drawing projecting the pits on the surface of the support for the lithographic printing plate on a surface parallel to the support with a surface apparent area of the support for the lithographic printing plate.
Roughness caused by asperities on the support surface is present on a photosensitive layer surface of a positive working presensitized plate of a thermal type. When the photosensitive layer is brought into contact with an object or the like, if the surface of the photosensitive layer is rubbed by the object or the like, a top part of a micro convex portion is slightly rubbed off, fracturing a surface slightly soluble layer, and even the support may be partially exposed. In development, developer easily infiltrates an interface between the support and the photosensitive layer from the fractured portion of the surface slightly soluble layer. Accordingly, the photosensitive layer starts dissolving from near the interface with the support. In other words, development is started preferentially from the rubbed place. Thus, a scratched portion is observed as a white line from a macroscopic standpoint.
The inventors obtained the foregoing knowledge as a result of serious studies. The inventors conducted further serious studies on measures to reduce a level of fine asperities on the surface of the photosensitive layer. As a result, it was discovered that a surface asperity shape on the surface of the support itself decided fine asperities on the surface of the photosensitive. It was also discovered that it was possible to reduce the level of fine asperities on the photosensitive layer surface without deteriorating press life thereof or the like by setting a ratio of a real area to a apparent area of the surface of the support in a specified range, specifying a pit structure, and setting a ratio of a pit apparent area to the surface apparent area in a specified range. Accordingly, the support for the lithographic printing plate capable of forming a photosensitive layer which has damage resistance was realized.
That is, an effective way to realize a flat photosensitive layer surface is to make a surface shape of the support flat as much as possible. However, since adhesion is lowered between the photosensitive layer and the support if the surface shape of the support is made simply flat, press life of the lithographic printing plate is deteriorated, peeling easily occurs between the photosensitive layer and the support, and damaged easily even in printing plate work. On the other hand, if a contact area between the photosensitive layer and the support is only increased simply by mechanical graining treatment or the like in order to increase adhesion between the photosensitive layer and the support, asperities are formed on the photosensitive layer surface, therefore, the photosensitive layer is damaged easily.
According to the present invention, adhesive is secured between the photosensitive layer and the support by setting the ratio of the real area to the apparent area of the surface of the support larger by 1.3 to 1.8 times. To form a smooth shape of the surface of the photosensitive layer while maintaining the ratio, the pit having the average diameter of 0.3 to 1.0 xcexcm, and the micro grained structure inside is provided on the surface and the ratio of the pit apparent area to the surface apparent area is set to 90% or more. Thus, it is possible to provide both press life or the like and damage resistance for the lithographic printing plate.
Preferably, the support for the lithographic printing plate has a large-medium-small complex grained structure with 3 different frequency undulations and the large grained structure (also referred to xe2x80x9cgrained structure with large undulationxe2x80x9d hereinafter) has a wavelength of 3 to 10 xcexcm, the medium grained structure (also referred to as xe2x80x9cgrained structure with medium undulationxe2x80x9d hereinafter) is the pit, and the small grained structure is the pit micro grained structure. With such a structure, the press life and water receptivity of the lithographic printing plate become more preferable.
The inventors also discovered scratches does not occur easily by forming a shape of the surface of the support in the following manner. That is, in order to increase a surface area of the support so as to secure adhesion between the photosensitive layer and the support while reducing asperities to make smooth the surface of the photosensitive layer, the surface of the support is provided with a grained structure with large undulation having a wavelength of 2 to 10 xcexcm, and a grained structure with medium undulation consisting of pits having an average diameter of 0.05 to 0.5 xcexcm by performing electrochemical graining treatment by alternating current electrolysis using electrolyte containing hydrochloric acid at 100 C/dm2 or lower of a quantity of electricity when the aluminum plate was at an anode side. Thus, a support for a lithographic plate according to a second aspect of the present invention was completed.
That is, the second aspect of the present invention provides a support for a lithographic printing plate obtained by performing graining treatment and anodizing treatment on an aluminum plate,
comprising a grained structure with large undulation having a wavelength of 2 to 10 xcexcm and a grained structure with medium undulation consisting of pits, each having an average diameter of 0.05 to 0.5 xcexcm on a surface thereof,
wherein the grained structure with medium undulation is obtained by performing electrochemical graining treatment by alternating current electrolysis using electrolyte containing hydrochloric acid at 100 C/dm2 or lower of a quantity of electricity when the aluminum plate was at an anode side, and chemical etching treatment to set a quantity of dissolved aluminum to 0.05 to 0.5 g/m2.
In the support for the lithographic printing plate thus constructed, when a photosensitive layer of a thermal positive working type is provided, a presensitized plate is realized, having limited asperities on a smooth surface of the photosensitive layer, a large surface area of the support. Accordingly, the presensitized plate is not damaged easily, excellent in printing performance, and handling is easy in conventional operation.
According to the present invention, preferably, the grained structure with medium undulation is obtained by carrying out chemical etching to set the quantity of dissolved aluminum to 0.05 to 0.5 g/m2 after the electrochemical graining treatment. The grained structure with medium undulation thus obtained by carrying out the chemical etching makes the surface of the support smoother, therefore the surface of the photosensitive layer smoother.
The inventors discovered that in order to secure adhesion between the photosensitive layer and the support by increasing a surface area of the support while reducing asperities to make smooth the surface of the photosensitive layer, by forming a shape of a large-medium-small complex grained structure consisting of 3 different frequency undulations, which has a grained structure with large undulation having a wavelength of 2 to 10 xcexcm, a grained structure with medium undulation consisting of pits having an average diameter of 0.1 to 1.5 xcexcm, and a grained structure with small undulation consisting of a micro grained structure inside a pit, scratches do not occur easily.
Further, only with the foregoing structure, it is difficult to remove the photosensitive layer having entered the micro grained structure inside the pits constituting the grained structure with small undulation. Thus, in order to compensate for this, developing performance (sensitivity) must be improved.
The inventors discovered that on the surface of the support having the shape of the above-described large-medium-small complex grained structure consisting of 3 different frequency undulations, by setting an average pore diameter and an average pore density on the anodized layer in specified ranges smaller than normal, it was possible to reduce the quantity of the photosensitive layer entering micropores and to prevent a reduction in an infiltration speed of the entire photosensitive layer caused by infiltration of developer into the micropores. Accordingly, the inventors discovered that it was possible to realize a presensitized plate, which has damage resistance, is high in sensitivity, and high in printing performance. Thus, a support for a lithographic printing plate according to a third aspect of the present invention was completed.
That is, the third aspect of the present invention provides a support for a lithographic printing plate obtained by performing graining treatment, alkali etching treatment and anodizing treatment on an aluminum plate,
comprising a grained structure with large undulation having a wavelength of 2 to 10 xcexcm, a grained structure with medium undulation consisting of pits, each having an average diameter of 0.1 to 1.5 xcexcm and a grained structure with small undulation consisting of a micro grained structure inside a pit on a surface thereof, and
with regard to an anodized layer formed by the anodizing treatment, an average pore diameter of micropores is 0 to 15 nm, and an average pore density is 0 to 400 pieces/xcexcm2.
The present invention also provides a presensitized plate comprising each of the supports for a lithographic printing plate according and a photosensitive layer that can become alkali-soluble by heating provided on the support. Since the presensitized plate of the present invention uses the support for the lithographic printing plate of the present invention, compared with the conventional positive working presensitized plate of the thermal type, it has better damage resistance, higher sensitivity, and better press life or the like when it is processed into a lithographic printing plate.
As described above, according to the present invention, it is possible to greatly improve tendency to be damaged, which has been a problem inherent in the presensitized plate of the thermal positive working type.